Each unit of SRM 2461 consists of a circular electroformed nickel plate about 1 mm thick, replicated from the head of a fired master cartridge case and cemented to a brass cylinder holder (see Figure 2) so that the assembly resembles an actual fired cartridge case. Standard Reference Material (SRM) 2461 is a physical standard that provides markings of a fired cartridge case. On the left, a NIST SRM 2461 Cartridge Case mounted on a brass cylinder holder right, optical micrograph showing the three certified areas of the SRM, breech face impression (BF), firing pin impression (FP), and ejector mark (EM). Surface topography profiles of the land engraved areas of the SRM 2460 Bullet may be found in the 2D Virtual SRM Database section of the web-based Surface Metrology Algorithm Testing System (SMATS). Comparison results showed high repeatability and reproducibility of bullet signatures of NIST standard bullets . Figure 1 shows one of the standard bullets produced by this fabrication process. The digitized bullet signatures were stored in a computer to control the NC diamond turning machine for the production of the standard bullets. The original bullet signatures were replicated from master bullets fired by a standardized shooting procedure at the National Laboratory Center of the ATF and at the Federal Bureau of Investigation Crime Laboratory. Specially designed fixtures were used to hold the standard bullets on the diamond turning machine for manufacturing of the bullet signatures. The numerically controlled (NC) diamond turning process was used at NIST for production of the standard bullets. These bullet signatures must be highly repeatable in different axial sections on the same standard bullet, and highly reproducible in a group of standard bullets. The bullet signature patterns of the standard bullets come from actual fired bullets. The standard bullets were designed to have size, shape, color, and material as close as possible to real bullets (see Figure 1). They have been tested in volunteer crime laboratories across the country for the purpose of developing quality control procedures for optical acquisitions in the National Integrated Ballistic Information Network directed by the Bureau of Alcohol Tobacco, Firearms, and Explosives (ATF) .

The NIST standard bullets, Standard Reference Material (SRM) 2460, and standard cartridge cases (SRM 2461) have been developed as reference standards for crime laboratories to help verify that the computerized optical-imaging equipment in those laboratories is operating properly and to facilitate ballistics measurement traceability and laboratory accreditation.

By analyzing these ballistic signatures, examiners are able to connect a particular firearm to criminal firearm investigations. As with fingerprints, every firearm has unique characteristics and, when fired, imprints unique signatures on the bullets and cartridge case.

Ejector marks